1. Field of the Invention
This invention relates to a communication system that improves upon time division systems using guard fields for passive compensation of propagation delays. In one specific embodiment, the invention relates to a device and method enabling reliable and more flexible and efficient synchronization of time division multiple access (TDMA) radio-frequency (rf) transmissions, including such transmissions from airborne platforms, through adaptive propagation delay compensation.
2. Description of the Related Art
Many wireless communications systems currently provide digital communications services to multiple users in accordance with a narrow band TDMA (NBTDMA) random access communications standard. Such standards include the North American Digital Cellular standard IS-136 (used in the U.S.), the Global System for Mobile Communications (GSM) standard (which is a non-U.S. standard for all-digital cellular communications), and the Japanese digital cellular standard. Some non-telephonic systems use NBTDMA formatted transmissions as well, such as various military and defense-related systems, wireless LANs, and NASA""s Advanced Communications Technology Satellite (ACTS) system.
An NBTDMA system provides communications services to multiple users through shared access to common narrow-band transmission channels. The system supports transmissions by several users on each channel by dividing the transmission time over the channel into a series of discrete frames. Each frame, in turn, includes a sequential series of data slots. The system waveform for the channel is thus comprised of a common number of data slots organized in each of a repeating series of discrete frames.
Several users can communicate simultaneously over a given channel because the system assigns to each user""s transceiver one or more data slots in the frame structure. A transceiver organizes its messages into series of burst transmissions and coordinates these bursts to coincide with its assigned data slots. Access to the channel therefore passes sequentially from one user to the next, in accordance with the pattern of data slot assignments.
All of the users of a NBTDMA channel therefore transmit over the same transmission frequency, but no two users transmit at the same time. Each avoids interference with the transmissions of the others by restricting its transmissions to coincide accurately with its data slots as assigned by the receiving base station. The accurate coordination of a transceiver""s transmissions, so as to coincide with its assigned transmission slots (as observed at the base station), constitutes timing synchronization.
The IS-136 format and other TDMA signaling formats have a principal objective of enabling multiple users to share simultaneous access to an assigned frequency channel through timing synchronization. TDMA synchronization is described in, for example, U.S. Pat. Nos. 5,177,740, granted Jan. 5, 1993 to Toy et al.; U.S. Pat. No. 5,283,815, granted Feb. 1, 1994 to Chennakeshu et al.; and U.S. Pat. No. 5,343,498, granted Aug. 30, 1994 to Toy et al.
Efficient use of available transmission spectrum requires a NBTDMA transmission format to use as little data capacity as possible for transmission overhead functions, such as synchronization. In turn, a system requires less data capacity for synchronization when the user transmissions, upon receipt at the base station, more precisely align with the users""respectively assigned data slots. The necessary degree of precision demands that the system even account for differences in signal propagation times between different transceivers. Because a transmitted signal requires more time to reach the base station if the transceiver is farther away, a longer propagation delay is encountered for the more distant transceiver""s transmissions.
Traditional NBTDMA systems have accommodated the existence of varying propagation delays by padding each data slot with a guard field. In accordance with the IS-136 standard, cellular TDMA random access transmissions use a nominal signaling rate of 48.6 kbps, which corresponds to a bit period of about 20.6 microseconds.
Each guard field (also referred to as a guard time) provides a delay time of six bit periods, or about 123 microseconds. The guard time lengthens the time interval allotted by a base station for receiving a burst transmission from a transceiver. As long as the propagation delay from a transceiver does not exceed this guard time, that transceiver""s transmissions will not overlap into data slots reserved for other users.
Although guard fields add to transmission overhead because they occupy transmission time that the system could otherwise use for user data, their inclusion has been necessary for synchronization in conventional TDMA systems. For example, a typical TDMA cellular system divides its service area into many overlapping cells. These cells may vary in size from one locality to another, with smaller cells used where user traffic requires more system capacity. Each cell has a base station that services calls placed by transceivers located within the cell.
These systems have minimized guard field overhead by restricting guard field length to the minimum number of bits necessary to accommodate their largest cells. That is, the guard field is limited in length, or time, to the minimum necessary to compensate for the propagation delay arising from the maximum range allowed between a base station and a transceiver within that base""s cell.
The efficacy of guard fields in ensuring synchronization therefore depends upon restricting transceivers to a specified maximum range from their currently assigned base stations. A ground-based transceiver accommodates this restriction inherently, because movement beyond the specified maximum range necessitates entry into a new cell, at which time the system reassigns the transceiver to the base station of the new cell. The guard fields continue to provide adequate guard times because, upon entry into the new cell, the transceiver lies within the specified maximum range to the new base station.
A problem with use of guard fields has arisen, however, as cellular system coverage has expanded. Cellular systems may be used to offer service to transceivers aboard aircraft. In addition, NBTDMA communication readily admits of application to missile systems, satellites in low Earth orbit, and lighter-than-air craft. Airborne platforms of this type constitute new and growing areas of application for NBTDMA technology. The guard field approach proves inadequate in each of these areas because the airborne platform is maintained at some substantial altitude above ground level while it occupies airspace over a given cell. The base station of that cell constitutes the system base station nearest the airborne platform.
Due to the platform altitude above ground level, the distance between the platform and the system base station transmission may exceed the specified maximum range achieved by ground-based transceivers. In the frequently-occurring worst case, the platform altitude alone exceeds the specified maximum range and the propagation delay always exceeds the guard time.
The minimum-length guard fields which are adequate for strictly ground-based systems therefore fail to allow transceivers aboard airborne platforms to time-synchronize with their base stations. A possible solution to this problem entails enlarging the guard fields, thereby providing longer guard times that accommodate longer propagation delays. However, use of larger guard fields would reduce system capacity, as indicated above. Moreover, transceivers may be carried at altitudes ranging from a few hundred feet to several miles or more. Guard fields large enough to accommodate such wide ranges would not be practical for commercial communications systems.
A delay compensation system and method are provided for enabling a transceiver aboard an airborne platform to properly align its transmissions with its assigned NBTDMA data slots. The system and method are backwards-compatible with communications systems implemented in accordance with existing NBTDMA transmission formats. This compensation system and method allows design of new NBTDMA formats that completely eliminate use of guard fields and their accompanying transmission overhead requirements.
The invention, which provides adaptive propagation delay compensation in a communications system, employs a range computation unit that receives a transceiver position data signal and a base position data signal, respectively representative of a mobile transceiver position and a base station position. The range computation unit generates a signal measuring line of sight range between the mobile transceiver and the base station assigned to the transceiver. The transceiver and base station communicate through a time division multiple access communication signal. A delay computation unit that receives the direct range measurement signal generates the required delay measured relative to the received communication signal so that the communication signal transmitted from the transceiver to the base station arrives in alignment with its assigned time slot.